According to conventional art, when the friction force between the road surface and a tire is reduced in such cases as when the road surface is wet in the rain, the vehicle can skid and move in an unexpected direction at the time when the accelerator is pushed down, causing an accident.
To prevent an accident caused by such skids, sudden acceleration, and soon, there have been developed Anti-Lock Brake System (hereinafter referred to as ABS), Traction Control System, and further a stability control system provided with a YAW sensor, and other systems.
For example, ABS is a system which senses the rotation state of each tire and controls the braking force based on the sensing result to prevent each tire from entering a locking state.
As the rotation state of a tire, it is possible to sense the number of rotations of each tire and the states such as air pressure and distortion, and use the sensing result for the control.
As examples of such control system, there have been known a brake device for an automobile (hereinafter referred to as Patent Document 1) disclosed in Japanese Patent Publication 5-338528), a brake control device (hereinafter referred to as Patent Document 2) disclosed in Japanese Patent Publication 2001-018775, a method and device for controlling a vehicle (hereinafter referred to as Patent Document 3) disclosed in Japanese Patent Publication 2001-182578, a vehicle movement control device (hereinafter referred to as Patent Document 4) disclosed in Japanese Patent Publication 2002-137721, a brake device (hereinafter referred to as Patent Document 5) disclosed in Japanese Patent Publication 2002-160616, and so on.
In Patent Document 1, there is disclosed a brake device in which negative pressure is supplied from a vacuum tank to a vacuum booster linked with a brake pedal, and negative pressure is supplied from a vacuum pump to the vacuum tank, and the vacuum pump is driven by a pump motor, whereby the pump motor is controlled so as to operate the vacuum pump when a state in which deceleration of an automobile reaches a predetermined value is detected by an acceleration sensor 14, thereby preventing the change of an operation feeling at the time of an abrupt brake operation as well as a brake operation immediately after that.
In Patent Document 2, there is disclosed a brake control device including control means which effects ABS control, the control means being provided with: lateral acceleration estimation means for estimating lateral acceleration generated in a vehicle; and comparison and determination means for comparing an estimated lateral acceleration by the lateral acceleration estimation means, an estimated lateral acceleration by vehicle behavior sensing means and a detected lateral acceleration sensed by a lateral acceleration sensor included in the vehicle behavior detecting means and for determining that normal turning corresponding to the rudder angle is under way if the difference between the estimated and sensed accelerations is less than a predetermined value and determining that abnormal turning is underway if the difference is equal to or larger than the predetermined value, whereby the control means is adapted to change the type of control based on whether normal turning or abnormal turning is determined to be under way during the ABS control.
In Patent Document 3, there is disclosed a method and device for controlling a vehicle in which a control signal for adjusting the deceleration and/or the acceleration of a vehicle is formed by a corresponding set value, wherein a correction factor expressing a vehicle acceleration or a vehicle deceleration produced by an inclination of the running road surface is formed, and the correction factor is made to overlap the set value, whereby the setting of deceleration and/or acceleration of the vehicle is improved.
In Patent Document 4, there is disclosed a vehicle movement control device in which skid angle change speed β′ of the center-of-gravity point as the actual yawing momentum of a vehicle having plural wheels is acquired, and brake fluid pressure ΔP is applied to one of brakes of left and right rear wheels when the absolute value of the change speed β′ is equal to or larger than predetermined value β0′,thereby generating the yawing moment in a manner in which the more the absolute value of the change speed β′ is, the more the value of the yawing moment is and at the same time, in a direction to reduce the absolute value of the change speed β′; even during the control of the yawing moment, determination of whether the slip control is needed or not on the wheel to which the brake fluid pressure ΔP is applied, continues and, when the slip control is needed, the slip control for keeping a slip ratio within a proper range is executed by controlling the brake fluid pressure ΔP.
In Patent Document 5, there is disclosed a brake device including at least two sensors from among an acceleration sensor for sensing an acceleration in the longitudinal direction of a vehicle, a wheel speed sensor for sensing a wheel speed of each wheel, and a braking pressure sensor for sensing a braking pressure, wherein a target braking pressure is calculated by use of feedback from at least two sensors, and an indicator current is calculated by an indicator current calculation section based on this calculation result, and the indicator current is carried into a brake drive actuator to produce braking force corresponding to the amplitude of the indicator current, whereby even when disturbance occurs or one sensor is in trouble, the abnormal output can be suppressed.
As a method for sensing the number of rotations of a tire, as shown in FIGS. 21 and 22, there is typically used a method which senses the number of rotations of a tire by use of a rotor A1 rotating integrally with a wheel carrier, and a pickup sensor A2. In this method, plural concaves and convexes spaced equally around the circumferential surface of the rotor A1 traverse the magnetic field generated by the pickup sensor A2, whereby the magnetic flux density is varied, generating a pulsative voltage in a coil of the pickup sensor A2; the number of rotations can be detected by sensing this pulse. An exemplary basic principle of this method is disclosed in Japanese Patent Publication 52-109981.
Patent Document 1: Japanese Patent Publication 5-338528
Patent Document 2: Japanese Patent Publication 2001-018775
Patent Document 3: Japanese Patent Publication 2001-182578
Patent Document 4: Japanese Patent Publication 2002-137721
Patent Document 5: Japanese Patent Publication 2002-160616
Patent Document 6: Japanese Patent Publication 52-109981
In the technique disclosed in Patent Document 1, however, while the brake control operation feeling is improved, when the friction force between the tire and road surface changes, it is difficult to set a threshold value for which there are assumed such cases as when the brake torque exceeds the friction force between the tire and road surface, thus causing a skid.
In the techniques disclosed in Patent Documents 2 to 5, compared to the above described technique disclosed in Patent Document 1, more advanced control is performed in which an acceleration of the vehicle itself during running of the vehicle is sensed and the brake control of the vehicle is performed based on this acceleration. However, the friction force between the tire and road surface varies even in the identical vehicle depending on the kind of tire installed in the vehicle and the air pressure thereof. Further, there are vehicles, such as a 4WD vehicle, in which drive control is performed separately for each tire. Consequently, even with the control method which takes into consideration the acceleration of the vehicle itself during its running, highly accurate control may be impossible.
To address the above problems, an object of the present invention is to provide a vehicle drive control system sensing highly accurately and easily accelerations generated in each tire and at the same time, sensing a tire ground contact pattern to thereby perform vehicle stability control, and a sensor unit thereof and a tire.